Chromium plating process



United States Patent 3,454,474 CHROMIUM PLATING PROCESS Roger M. Woods, Washington, D.C., and David R. Moul, Seat Pleasant, Md., assignor to Corillium Corporation, Arlington, Va., a corporation of Virginia No Drawing. Filed Nov. 23, 1966, Ser. No. 596,425 Int. Cl. C23b /06 U.S. Cl. 204-51 5 Claims This invention relates to a highly eflicient process for producing a hard, dull, non-reflective, strongly adherent, electrodeposited coating of black chromium upon a conductive base material such as a metal.

In the prior art, attempts have been toward producing an electrodeposit, black in color, by employing such plating baths as those containing chromic anhydride and acetic acid or fluoride bearing ions. It has been found in using these baths that it was necessary to operate them at temperatures as low as 50 F., and to deposit films thereon at very high current densities of the order of one thousand to two thousand amperes per square foot. The

efiiciency of these baths has been found to be low, due,

in part, to the necessity of cooling the baths by means of supplemental refrigeration equipment. It has also been impractical to operate such processes on a production scale because of the high current requirements. Moreover, the films produced by these processes have been of rather inferior quality, being generally poorly adherent and of a lighter color than those obtainable in accordance with the present invention. Also, the abrasion resistance has been very poor, partly due to the fact that the bath efficiency fell off sharply shortly after the plating had begun, resulting in a very thin deposit.

It is an object of this invention to produce a process of electrodepositing an extremely adherent, matte or satin black chromium coating upon a conductive base, which coating is highly resistant to corrosion and exhibits excellent wear resistance and which protects the base material from corrosion.

Another object of the invention is to produce a process for finishing trim components for the purpose of reducing glare, particularly on automotive parts such as windshield wipers, hood ornaments, steering wheels, and dashboard hardware, with a coating which has an attractive and unique decorative appearance and which has excellent wearing properties. Another object of the invention is to provide a process using conventional chromium plating equipment wherein certain chemical compounds and conditions are provided for obtaining a black chromium late. p Other objects and advantages of the invention will become readily apparent from considering the following detailed description of the preferred embodiment of the invention.

According to the invention, a black chromium plate is obtained on a conductive base material in a system in which the base metal is cathodic and in which the electrolyte comprises an aqueous solution of chromic anhydride (CrO containing phosphoric acid and ethylenediaminetetraacetic acid in the solution. The solution contains from about 112 grams to 400 grams per liter of chromic anhydride and from about 1.5 grams to 15 grams per liter of phosphoric acid (H PO 85%), and from about 3 grams to 25 grams per liter of ethylenediamine tetraacetic acid (E.D.T.A.). The ethylenediamine tetraacetic acid is preferred, although its salts can be used such as sodium ethylenediajmine tetraacetate. It is necessary to stay within the above stated ranges in order to produce a suitable black plate.

The object to be plated is prepared for plating by conventional plating procedures, which may include degreas- "ice ing alkaline cleaning, acid dipping, polishing, sandblasting, etc.

The preferred electroplating bath comprises an aqueous solution containing 175 to 300 grams per liter of chromic anhydride and 4 to 8 grams per liter of phosphoric acid, and 5 to 10 grams per liter of ethylenediamine tetraacetic acid. Finally, 7.5 grams per liter of abarium compound, preferably barium carbonate, is added to the above Solution to precipitate and sulfate that might be present.

The plating bath is prepared in the concentrations as stated above and added to a plating tank to the desired operating level. The tank must be thoroughly cleaned to prevent bath contamination, particularly where sulfate or fluoride type baths have previously been used. Anodes are then placed in the tank and electrolyzed with dummy cathodes for about 10 minutes. The solution is frequently stirred during this period as by air agitation. The bath is then ready for use. The object or part to be plated is then immersed in the bath and is made the cathode of the electro-plating system. Anodes of various materials may be employed, but it has been found preferable to use lead or steel anodes. Carbon anodes may be employed, but with less satisfactory results. The tank which holds the electrolyte solution is preferably lined with a synthetic resin material such as polyvinylchloride or other suitable invert material. The plating tank should also be provided with an exhaust system to carry away fumes of the chromic acid.

The plating bath should be operated at a temperature of about 60110 F. and preferably about F. It was found that the particular temperature used within these ranges has little effect on the color of the deposit. The best deposits will usually be obtained at a cathode current density of between and 1000 amps per square foot, and preferably about 300-500 amperes per square foot.

Plating times will vary according to the thickness desired and complexity of the object to be plated, but normally the time will vary between about 30 seconds to about 10 minutes for decorative applications, and longer where thicker deposits are required.

After the object has been plated, it is removed from the bath and rinsed in a conventional manner and is allowed to dry. The covering power is approximately that of conventional chromium plating, and in cases of extremely deep recesses, internal anodes may have to be used, the same as in conventional chromium baths.

It has been found desirable to maintain the anode surface area as large as possible with respect to the cathode surface area in order to maintain the anode current density at a relatively low value to avoid coating the anode with a film, the exact nature of which is not fully understood, but which tends to increase the resistance of the system. The anode area should be as great as and preferably three or four times as great as the cathode area for most efiicient operation.

The plating bath may be replenished by the addition of chromic anhydride and'phosphoric acid and ethylenediamine tetraacetic acid as they are consumed to bring the concentrations of the ingredients back to the concentrations originally present in the bath. A sufiiciently accurate control for production use may be achieved by conventional analytical techniques.

The plating bath of the invention which produces a uniform, matte, black chromium finish is applicable to a wide variety of conductive base materials such as copper, nickel, steel, stainless steel, brass and bronze. Other metals such as zinc die cast and aluminum can also be plated after a suitable underplate is applied.

The plate produced by the above described process is matte and black in color and exhibits a non-reflective appearance when plated over most any substrate. Specular reflectance of the plated article may be reduced to an absolute minimum by rendering the exterior article matte in appearance prior to the final plating operation. There is no limitation to the thickness than can be produced by this process, merely by increasing the time in the plating bath.

It will be appreciated by those skilled in the art that this process overcomes the difficulties experienced in previous black chromium plating baths, in that they could not achieve a continuous rate of plating buildup beyond a few millionths of an inch thick or several tenthousandths at most. Samples plated by the above described process have achieved thicknesses of over thousandths of an inch with no sign of a limitation.

EXAMPLES Example 1.A conventional polyvinlychloride lined steel tank containing a carbon steel anode is used in this example and in the following examples. A stainless steel windshield wiper arm was prepared for plating according to conventional procedures as heretofore described. After rinsing, the arm was transferred to the steel tank which contained the black plating bath which comprised a solution containing 225 grams per liter of chromic anhydride and 6 grams per liter of phosphoric acid, and 8 grams per liter of ethylenediamine tetraacetic acid and 7.5 grams per liter of barium carbonate. The current density was 350 amperes per square foot and the temperature of the bath was 90 F. The plating time was five minutes. The arm was then removed and rinsed in cold water, then hot water, and dried. The arm has a coating of black chromium which is matte in appearance.

Example 2.A zinc die cast dashboard panel was prepared for plating as in the above example and then copper and nickel plated according to conventional procedures. Thereafter, it was rinsed and transferred to the black plating bath which contained 200 grams per liter of chromic anhydride and 4 grams per liter of phosphoric acid, and 6 grams per liter of ethylenediamine tetraacetic acid and 7.5 grams per liter of barium carbonate. The current density was 250 amperes per square foot and the temperature was 85 F. The plating time was three minutes. The panel was then removed and rinsed in cold water, then hot water, and dried. The panel has a uniform matte black coating of chromium.

Example 3.-A steel shaft was prepared for plating according to conventional procedures as heretofore described. After rinsing, the shaft was transferred to the black plating bath which comprised a solution containing 175 grams per liter of chromic anhydride and 6 grams per liter of phosphoric acid and 8 grams per liter of ethylenediamine tetraacetic acid, and 6 grams per liter of barium carbonate. The current density was 450 amperes per square foot and the temperature was 85 F. The plating time was one hour. The shaft was then removed and rinsed in cold water, then hot water, and dried. The shaft had a dull black coating that measured .002" thick.

In accordance with the provisions of the patent statutes we have explained the principle and mode of operation 4 of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

What we claim is:

1. A process of producing electrodeposited matte black chromium coatings comprising plating an object in an aqueous bath comprising about 112 grams to about 400 grams per liter of chromic anhydride, and about 1.5 to about 15 grams per liter of phosphoric acid, and about 3 grams to about 25 grams per liter of ethylenediamine tetraacetic acid, using a current density of about 150 to 1000 amperes per square foot at the cathode and a temperature of about 60 F. to about 110 F. for a period of time sufficient to give a chromium coating of desired thickness.

2. The process of claim 1 wherein said bath comprises about 175 to about 300 grams per liter of chromic anhydride and about 4 to about 8 grams per liter of phosphoric acid and about 5 to about 10 grams per liter of ethylenediamine tetraacetic acid.

3. The process of claim 1 wherein said current density is about 300 to about 500 amperes per square foot.

4. The process of claim 1 wherein said temperature is about to F.

5. A process of producing electro-deposited matte black chromium coatings comprising plating an object in an aqueous bath comprising about 112 grams to about 400 grams per liter of chromic anhydride, and about 1.5 to about 15 grams per liter of phosphoric acid, and an amount of sodium ethylenediamine tetraacetate equivalent to about 3 grams to about 25 grams per liter of ethylenediamine tetraacetic acid, using a current density of about 150 to 1000 amperes per square foot at the cathode and a temperature of about 60 F. to about F., for a period of time suflicient to give a chromium coating of desired thickness.

References Cited UNITED STATES PATENTS 1,975,239 10/1934 Ungelenk et a1. 20451 1,985,308 12/1934 Bornhauser 204-5l 2,623,847 12/1952 Gilbert et a]. 204-51 2,739,108 3/1956 Quaely 20443 2,739,109 3/1956 Quaely 204-43 2,824,829 2/1958 Quaely 20443 FOREIGN PATENTS 624,887 8/1961 Canada.

OTHER REFERENCES Shenol, B. A. et al.: Metal Finishing, vol. 64, No. 4, pp. 46-52, April 1966.

Grifiin, John L.: Plating, vol. 53, No. 2, pp. 196-203, February 1966.

JOHN H. MACK, Primary Examiner.

G. L. KAPLAN, Assistant Examiner. 

1. A PROCESS OF PRODUCING ELECTRODEPOSITED MATTE BLACK CHROMIUM COATINGS COMPRISING PLATING AN OBJECT IN AN AQUEOUS BATH COMPRISING ABOUT 112 GRAMS TO ABOUT 400 GRAMS PER LTER OF CHROMIC ANHYDRIDE, AND ABOUT 1.5 TO ABOUT 15 GRAMS PER LITER OF PHOSPHORIC ACID, AND ABOUT 3 GRAMS TO ABOUT 25 GRAMS PER LITER OF ETHYLENEDIAMINE TETRAACETIC ACID, USING A CURRENT DENSITY OF ABOUT 150 TO 1000 AMPERES PER SQUARE FOOT AT THE CATHODE AND A TEMPERATURE OF ABOUT 60*F. TO ABOUT 110*F. FOR A PERIOD OF TIME SUFFICIENT TO GIVE A CHROMIUM COATING OF DESIRED THICKNESS. 